MIKE 21 BW, User Manual, Page: 95 / 196

1DH Boussinesq Wave Module - Examples
95
the regular waves the wave height is H = 1 m and the wave period T =8 s. For
the irregular waves the significant wave height is H
m0
= 1 m and the spectral
peak period T
p
= 8 s. The waves are synthesised based on a mean JON-
SWAP spectrum. The minimum wave period T
min
= 4 s. Hence the problem
can be solved using the enhanced Boussinesq type equations with a disper-
sion coefficient of B= 1/15.
The thickness of the porosity layer (8 point wide) is about one-quarter of the
wave length and the porosity value is set to 0.70. From the reflection-porosity
curve shown in Figure 4.63 it is seen that a porosity of 0.70 corresponds to a
reflection coefficient of about 0.4, assuming a characteristic wave height and
wave period of 1 m and 8 s, respectively.
The model domain is discretized, using a structured mesh with 195 elements
and 196 nodes. The mesh size is 2 m and the integration time step is 0.1 s
corresponding to a maximum Courant number of about 0.5 s. The simulation
duration is 5 minutes (3001 time steps).
Figure 4.63 Reflection coefficient versus porosity for a 16 m wide absorber in 10 m
water depth. The characteristic wave height and wave period is 1 m and
8 s, respectively. Calculated by use of the MIKE 21 Toolbox program
Calculation of Reflection coefficient
Model results
Time series of the simulated surface elevation extracted at point P(150) is
shown in Figure 4.64 for irregular waves and regular waves. In case of regu-
lar waves it is seen that the resulting wave height is increased due to the
wave reflection. It is more difficult to identify the increased wave height in
case of irregular incident waves.